Various types of punch and die assemblies have been devised in the past in order to join two sheets of material, such as metal, by means of a localized, interlocking deformation, sometimes referred to as a stitch. Stitches may be of a lanced type in which either one or both sheets of the metal are pierced, or may be of a so-called "leak-proof" type as disclosed in U.S. Pat. No. 4,459,735 issued on July 17, 1984 to Sawdon.
Punch and die assemblies used to form stitch joints of the type described above typically include an anvil body and a pair of die portions which are pivotally mounted on the anvil body. One or more springs are employed to bias the die portions toward each other. The die portions are forced away from each other upon engagement therewith by the punch which forces portions of the sheet metal between the die portions and into engagement with an anvil. The relatively high impact forces applied to the die assembly by the punch and the normally high repetition rate of production operations imposes extraordinary forces on the component parts of the die assembly. Particularly susceptible to failure are the springs which bias the die portions together and the pins which pivotally mount the die portions on the anvil body. Failure of the above-mentioned components in a production environment results in down-time in order to exchange dies and is also costly from the standpoint of repairing the dies.